1. Field of the Invention
The present invention relates to an exposure apparatus for holding, e.g., a wafer as a photosensitive substrate on a wafer stage through a wafer holder and exposing a reticle pattern onto the wafer and, more particularly, to an exposure apparatus having a cleaning means for cleaning foreign substances on the wafer holder.
2. Related Background Art
When a semiconductor element, a liquid crystal display element, or the like is to be manufactured by a photolithographic process, a projection exposure apparatus is used, which is designed to expose a pattern of a photomask or reticle (to be referred to as a reticle hereinafter) onto a wafer on a wafer stage through a projection optical system. In such a conventional projection exposure apparatus, a wafer is chucked and held by a wafer holder mounted on a wafer stage so as to firmly hold the wafer in a horizontal state.
If, however, the wafer is chucked while foreign substances such as dust are present between the wafer holder for holding the wafer and the wafer, the horizontal state of the exposure surface of the wafer is degraded by the foreign substances. The degradation in the horizontal state of the exposure surface causes a positional error and a focus error in each shot area of the wafer and hence is a major factor that decreases the yield in the manufacture of LSIs and the like. For this reason, in general, an exposure process is stopped every predetermined interval, and the wafer holder is moved to a position where the operator can reach the holder so as to allow the operator to wipe the overall wafer holder with his/her hand by using a whetstone or a dust-free cloth.
FIG. 5 shows the manner of a conventional wafer holder cleaning operation. Referring to FIG. 5, a stationary mirror 2X is fixed to a side surface of a projection optical system 1 in one direction (X direction), and a stationary mirror 2Y is fixed to a side surface of the projection optical system 1 in the Y direction perpendicular to the X direction. A wafer holder 3 is disposed below the projection optical system 1, and the wafer holder 3 is fixed on a wafer stage 5. The wafer holder 3 is a thin columnar member and having a plurality of concentric grooves 4 formed in its surface. A wafer is placed on the wafer holder 3, and a negative pressure is applied to the wafer by a vacuum pump through a large number of suction holes (not shown). As a result, the wafer is chucked and held by the wafer holder 3.
The wafer stage 5 is constituted by an XY stage, a Z stage, and the like. The XY stage can position a wafer at an arbitrary position within the movable range of the XY stage on a plane (XY plane) perpendicular to the optical axis of the projection optical system 1. The Z stage serves to position the wafer in the optical axis of the projection optical system 1. In order to perform positioning within the XY plane, an X-axis movable mirror 6X and a Y-axis movable mirror 6Y are fixed on the wafer stage 5. A laser beam LB1 is radiated from an X-axis laser interferometer (not shown) onto the stationary mirror 2X of the projection optical system 1, and a laser beam is also radiated parallel to the laser beam LB1 onto the movable mirror 6X. The X-axis laser interferometer always monitors the coordinates of the wafer stage 5 in the X direction with reference to the projection optical system 1. Similarly, a laser beam LB3 is radiated from a Y-axis laser interferometer (not shown) onto the stationary mirror 2Y of the projection optical system 1, and a laser beam is also radiated parallel to the laser beam LB3 onto the movable mirror 6Y. The Y-axis laser interferometer always monitors the coordinates of the wafer stage 5 in the Y direction with reference to the projection optical system 1.
When the wafer holder 3 is to be cleaned, an end portion of the wafer stage 5 is moved to a position below the projection optical system 1, and the wafer stage 5 is fixed while the wafer is kept removed from the wafer holder, as shown in FIG. 5. A cleaning tool 7 constituted by a whetstone or a dust-free cloth is placed on the wafer holder 3. The operator then moves the cleaning tool 7 on the entire surface of the wafer holder 3, as indicated by a locus 9, while the cleaning tool 7 is biased against the wafer holder 3 with a constant pressure by usin.sub.G a hand 8, thereby removing foreign substances on the wafer holder 3.
If, however, the wafer holder 3 is manually cleaned by the operator as in the prior art, it takes about 30 minutes to one hour to wipe the entire surface of the wafer holder 3, thus shortening the actual exposure time. As a result, the throughput is decreased. If the wafer holder 3 is cleaned once or so per day in order to prolong the exposure time, the possibility that exposure will be performed with respect to a wafer while foreign substances are chucked on the wafer holder 3 is increased, resulting in a reduction in the yield of semiconductor elements and the like.
In addition, if a cleaning operation is manually performed by the operator, the surface of the wafer holder 3 may be partly left unwiped, and new foreign substances from the operator may adhere to the wafer holder 3.
In order to solve these disadvantages, for example, Japanese Laid-Open Patent Application No. 5-82411 proposes a technique of automatically cleaning the wafer holder using a cleaning member. Additionally, to determine the presence/absence of foreign substances such as dust, U.S. Pat. No. 4,849,901 proposes a method in which the previous focus position in a shot area is compared with the current focus position in the shot area by a focus check mechanism for detecting the focus position of a wafer, thereby determining the presence/absence of foreign substances. However, in the conventional method of determining the presence/absence of foreign substances, exposure is performed to an intermediate shot area, and an alarm is generated at a shot position where foreign substances are confirmed to be present. For this reason, if foreign substances are present, the cleaning process must be repeated. Therefore, even if the cleaning operation is automatically performed in the cleaning process, the throughput of the entire exposure process cannot be improved although the yield of finished products can be increased.